The present invention generally relates to a transparent electrode for optoelectronic devices. In particular, the present invention relates to a transparent nanowire electrode for optoelectronic devices.
Producing optoelectronic devices on flexible substrates is an important field of research nowadays. Using organic materials represents an important step for realizing same. Electrodes are necessitated for external contacting, of which at least one needs to be transparent. These electrodes are to satisfy special requirements: high conductivity is necessitated, and at the same time high transparency in the visible wavelength range. These electrodes also need to exhibit constant optical and electrical characteristics when stressed (bent) mechanically. Furthermore, producing these electrodes is to be cheap and scalable on a large scale, for example transferrable to roll-to-roll methods, and use materials of sufficient availability. Flexible electrodes available at present do not succeed in fulfilling all the requirements at the same time.
The state-of-the-art material here is indium tin oxide (ITO) which exhibits a low sheet resistance RS at, at the same time, high transparency T. However, this material is brittle so that it is only suitable for being applied on flexible substrates to a limited extent. [Kim2009] presents a flexible ITO electrode which, however, comprises a low conductivity (RS˜42 ohm/sq), which is reduced further when stressed mechanically.
A promising alternative to ITO for the transparent electrodes are metal nanowires made of copper (CuNW) or silver (AgNW) which are applied onto glass or film substrates as a percolation network of arbitrary orientation [Peumans2008], [Alden2007]. Electrodes having wires made of silver are also referred to as AgNW electrodes. They exhibit high conductivity and transmission, easy processing (for example spray coating processes) and very good flexibility characteristics. However, when producing these layers, a post-processing step is necessitated in order to reduce the contact resistances between the nanowires and, thus, obtain a low sheet resistance.
At present, thermal post-treatment (about 90 minutes at 210° C. or longer a period at lower temperatures) is, for example, performed in order to reduce the sheet resistance [Sachse 2013]. Post-treatment (for example heat acting on it) of the electrodes is a particularly large obstacle since this makes processing considerably more complicated. In addition, this step makes application on flexible substrates more difficult since many polymer films lose their flexibility or are destroyed at such temperatures.
In [Hu2010], Hu et al. show a flexible electrode having a low sheet resistance of 12 ohm/sq which was processed at 120° C., but low transmission (about 60% with a wavelength of 550 nm).
In [De2009], DE et al. present flexible AgNW electrodes exhibiting good optical and electrical characteristics (T=85% with no substrate, RS=13 ohm/sq), however producing thereof is complicated (transfer process of the AgNWs) and difficult to scale.
In [Choi2013a], Choi et al. show a simple 2-step spray process. An AgNW electrode is produced here by means of spray coating, which subsequently is covered with PEDOT:PSS at 60° C.—also in a spray coating method. The resulting electrode does not require additional heat to be supplied and exhibits a low sheet resistance of RS=10.74 ohm/sq with a transmission of T=84%. These good values are achieved by the fact that the nanowires, when applying the PEDOT:PSS, are deformed at the contact points such that the contact area between nanowires placed one above the other is increased. In addition, PEDOT:PSS itself is conductive so that both the nanowires and, to a small extent, the PEDOT:PSS contribute to the conductivity of the electrode. However, the big disadvantage in [Choi 2013a] is that PEDOT:PSS needs to be used as an overlayer for deforming the nanowires. Additionally, PEDOT:PSS is acidic and slowly affects the nanowires so that such electrodes exhibit a short lifetime. A further disadvantage of PEDOT:PSS is the undesired parasitic absorption entailed in this layer.